During deposition in a processing chamber, film can be deposited not only on a frontside of a wafer, but also on a backside of the wafer. For example, in atomic layer deposition (ALD), a film can be deposited layer by layer by successive dosing and activation steps. In ALD processing chambers, precursor gases can be directed to a wafer and the precursor gases can chemisorb onto a surface of the wafer to form a monolayer. Additional precursor gases can be introduced that react with the monolayer, and a purge gas can be subsequently introduced to remove excess precursors and gaseous reaction by-products. Precursor gases can be alternately pulsed without overlap, and cycles can be repeated as many times as desired to form a film of suitable thickness.
However, during the deposition process, unwanted process gases, such as precursor gases in ALD, can be deposited on the backside of the wafer. Since ALD is a surface-based deposition process, film can be deposited on any accessible surface in the processing chamber. Thus, any gap accessing the backside of the wafer can permit process gases to flow to the backside and then deposit onto the backside of the wafer. Film on the backside can be generated by transport of precursor gases during the dose step, and the reaction of the precursor gases can occur during the activation step. In some implementations, a ring of film as thick as the frontside film at the outer edge of the wafer can extend to greater than 5 mm inward from the outer edge.
One of the drawbacks of backside deposition is that it can lead to alignment/focusing issues during lithography. To apply a desired pattern onto a target portion of the wafer, various tools in lithography can be used for proper alignment and focusing for the desired pattern. After a deposition step, if film is deposited on the backside of the wafer, then the various lithography tools may require re-adjustment in focus and alignment. This can lead to unwanted lithography traces during patterning and increased time spent re-calibrating the various lithography tools.